LTC3115-1
17
Rev. C
OPERATION
input undervoltage lockout threshold. The buck-boost con-
verter is enabled when the RUN pin reaches 1.21V which
allows the rising UVLO threshold to be set via the resis-
tor divider ratio. Once the RUN pin reaches the threshold
voltage, the comparator switches and the buck-boost
converter is enabled. In addition, an internal 0.5µA (typi-
cal) current source is enabled which sources current out
of the RUN pin raising the RUN pin voltage away from
the threshold. In order to disable the part, V
IN
must be
reduced sufficiently to overcome the hysteresis generated
by this current as well as the 100mV hysteresis of the
RUN comparator. As a result, the amount of hysteresis can
be independently programmed without affecting the rising
UVLO threshold by scaling the values of both resistors.
THERMAL CONSIDERATIONS
The power switches in the LTC3115-1 are designed to
operate continuously with currents up to the internal cur-
rent limit thresholds. However, when operating at high
current levels there may be significant heat generated
within the IC. In addition, in many applications the V
CC
regulator is operated with large input-to-output voltage
differentials resulting in significant levels of power dis-
sipation in its pass element which can add significantly to
the total power dissipated within the IC. As a result, care-
ful consideration must be given to the thermal environ-
ment of the IC in order to optimize efficiency and ensure
that the LTC3115-1 is able to provide its full-rated output
current. Specifically, the exposed die attach pad of both
the DHD and FE packages should be soldered to the PC
board and the PC board should be designed to maximize
the conduction of heat out of the IC package. This can be
accomplished by utilizing multiple vias from the die attach
pad connection to other PCB layers containing a large area
of exposed copper.
If the die temperature exceeds approximately 165°C, the
IC will enter overtemperature shutdown and all switching
will be inhibited. The part will remain disabled until the
die cools by approximately 10°C. The soft-start circuit is
re-initialized in overtemperature shutdown to provide a
smooth recovery when the fault condition is removed.
Figure 3. Accurate RUN Pin Comparator
1.21V
0.8V
V
IN
V
IN
0.5µA
LTC3115-1
ENA
31151 F03
RUN
R1
R2
ENABLE
SWITCHING
INPUT LOGIC
THRESHOLD
ENABLE
V
CC
REGULATOR AND
CONTROL CIRCUITS
+
–
+
–
